Thirty-Year Period in Secular Variation of the Main Geomagnetic Field

TL;DR

This study identifies a consistent 30-year periodic pattern in the secular variation of the main geomagnetic field over the 20th century, driven mainly by non-dipole components.

Contribution

It reveals a three-episode 30-year cycle in geomagnetic variation and emphasizes the role of non-dipole fields over dipole fields in this periodicity.

Findings

Three main episodes of maximum annual rates around 1910-1920, 1940-1950, and 1970-1980.

Shorter rising phases compared to declining phases in each episode.

Non-dipole fields govern the periodic variation, despite dipole dominance in magnetic energy.

Abstract

During the centennial period from 1900 to 2000, the globally averaged unsigned annual rate of the main geomagnetic field experienced a three-episode variation. The maximum annual rates occurred respectively around 1910-1920, 1940-1950, and 1970-1980, showing a 30-year period. In addition, the rising phase in each episode is much shorter than declining phase. The governing factor of this periodic variation is non-dipole field, instead of the dipole field, although the secular variation of the magnetic energy concerned with the dipole is dominative over all other multiples.